Medication errors in the ambulatory setting are common, occurring in approximately 8-21% of prescriptions. Adverse drug events (ADEs) in the ambulatory setting are also common, affecting 25% of patients. Of those ADEs in the ambulatory setting, 11% are preventable (i.e. could have been avoided entirely) and 28% are ameliorable (i.e. could have had severity or duration substantially reduced). Increasing evidence indicates that health information technology (HIT) decreases medication errors in the inpatient setting, but less evidence is available in the outpatient setting or during transitions from inpatient to outpatient settings. Moreover, most of the literature documenting the success of HIT interventions has been driven by a few institutions that developed home-grown systems iteratively over several decades. The ability to generalize those findings to commercial systems that are more accessible to other institutions or providers is unclear. Another largely unaddressed issue is the organizational and safety effects of institutions transitioning from legacy electronic systems to newer applications. We will conduct 3 studies in order to measure the impact of HIT on patient safety in the ambulatory setting, specifically 2 sites in the Ambulatory Care Network of New York-Presbyterian Hospital. The first study will be a multi-center before-and-after prospective study measuring the effects on medication errors of transitioning from one electronic prescribing system to another, in this case from home-grown to vendor-based systems. The second study will be a qualitative study of physicians, using one-on-one-interviews and direct observation to understand variations in human-computer interactions with the new electronic prescribing systems and subsequent correlation with medication error rates of individual physicians. The third study will be a randomized controlled trial evaluating electronic transmission of discharge medication lists from the hospital to the ambulatory setting. This study will examine: 1) the impact of the intervention on medication discrepancies at the first ambulatory visit following discharge (comparing the discharge medication list to the "current" medication list recorded by the ambulatory physician during follow-up) and 2) ADEs 30 days post-discharge. The proposed studies will evaluate the effectiveness of 2 electronic interventions to decrease medication errors in the ambulatory setting. If effective, these interventions could be implemented and sustained in many other centers. Our studies also have implications for institutions or practices that are transitioning from one electronic health record to another, even if that transition is not from a home-grown to a vendor-based system, but rather from one vendor-based system to another. This proposal has the potential to yield critical insights into why certain HIT interventions work (or do not) and how future interventions should be designed to align themselves better with physicians'workflow. If rates of errors vary greatly between commercial systems, there are potential policy implications for more stringent certification criteria of electronic prescribing to ensure medication safety. This proposal will evaluate the effectiveness of 2 electronic interventions for decreasing medication errors in the ambulatory setting: vendor-based electronic prescribing systems and electronic transmission of discharge medication data from the inpatient to the outpatient settings. We will evaluate these interventions using a multi-center before-and-after prospective cohort study, a multi-faceted qualitative study, and a randomized controlled trial. If found to be effective, these interventions could be implemented in many other centers.